1. Field of the Invention
The present invention relates to small diameter metal tubing. More specifically, the present invention relates to creating a metal tube product that has a wall thickness that is substantially increased over conventional single tube manufacturing capabilities, as well as the method of manufacturing the metal tubing.
2. Description of the Related Art
Metal tubing is important in many industrial applications such as a protective shield for optical fibers and coated copper conductors. Tubing typically is measured by outer diameter and wall thickness. Manufacturing the tubing to high wall thicknesses to withstand crush or tensile forces is difficult because forming and welding metal above a certain wall thickness can cause excessive wear to the manufacturing machine and dramatically reduce the speed at which the tubing machine may operate.
On mills that use a laser welding process, for example, the ability to weld, maintain a speed that is an acceptable rate and draw or roll reduce is limited by the thickness of the metal. To double the wall thickness on a laser welding tube machine from 0.008 inches to 0.016 inches will reduce the speed dramatically and create significant wear and tear on the machine as the machine may not be optimally designed for this thickness of metal.
U.S. Pat. No. 4,759,487, hereby incorporated in its entirety by reference, discloses an apparatus and method for continuously manufacturing optical fiber cable in which the optical fibers are encased in a sealed metal tube formed from a flat metal strip as seen in FIGS. 1 and 2. The apparatus 10 creates a partially formed tube 12 from a steel strip 11 by passing the steel strip 11 through a first roller assembly 24. Optical fibers 13 are placed within the partially formed tube 12. The apparatus then closes the tube by a second roller assembly 65, and a welding laser 74 seals the tube. The sealed tube is then pulled through wire dies 95 by wheel 98 to reduce the diameter of the sealed tube as seen in FIG. 2.
FIG. 3 shows a plan view of a metal tube 300 created by the above process. As can be seen the tube includes a single wall 310.
As the diameter of a tube goes down and the wall thickness increases, the ability to form the metal strip to a round shape becomes increasingly difficult. In addition, if the tube is to be drawn or roll reduced down to a smaller size, the tube itself may be unable to withstand the forces of drawing or roll reduction. The equipment also may not be suitable to accomplish the task thus requiring a re-design of the equipment or completely new equipment.
Further, metal tubing created by the process has the potential for un-detected minor flaws in the weld area. These minor flaws, under adverse conditions such as pressure or corrosive materials can result in a product failure.